Fluids preferentially migrate into high permeability zones relative to low permeability zones in subterranean formations. This migration is undesirable when injecting treatment fluids into hydrocarbon-bearing formations for post-primary recovery of residual hydrocarbons. The treatment fluids channel through the relatively highly permeable zones bypassing the adjacent relatively less permeable zones. The result is poor conformance and undesirable flow profiles of the treatment fluid in the formation. Consequently, the hydrocarbons residing in the less permeable zones are not produced and the overall yield of hydrocarbons from the formation is reduced.
A solution to this problem is to improve conformance in the formation by reducing the permeability of the highly permeable zones so that treatment fluids are diverted away from the zones of reduced permeability into adjacent hydrocarbon-bearing zones, thereby facilitating recovery of hydrocarbons from the formation.
A number of processes are known for reducing the permeability of highly permeable zones in subterranean hydrocarbon-bearing formations. Polymer gels are often used as permeability reducing compositions in highly permeable zones. For example, high molecular weight polymers are injected into a formation along with a cross-linking agent. The agent cross links the polymers in situ to form a viscous, permeability reducing polymer gel. Gelation may be delayed until the injected fluids are in place by spaced sequential injection of the gel components or complexing the cross-linking agent with a retarding anion or in a redox system.
U.S. Pat. No. 4,039,029 to Gall teaches complexing a multivalent cation cross-linking agent with a retarding anion. The complex prevents gelation of the polymer during injection until the complex dissociates in the formation. "Control of Water Mobility Using Polymers and Multivalent Cations" by Needham et al, SPE Paper No. 4747, injects partially hydrolyzed polyacrylamide with aluminum citrate. The citrate sequesters the aluminum cation until the gel components are in place in the formation. The aluminum cation is freed from the complex over time to cross link the polymer. The resulting gel reduces the permeability of the porous media to water.
The complexing mechanisms used in situ to delay gelation are ineffective because the complexes are extremely sensitive to formation conditions, i.e. temperature and pH. If the formation conditions are unfavorable, the cross-linking agent is not released from the complex in a reactive form and is incapable of producing a permeability reducing gel. Therefore it is difficult to create a stable gel which effectively reduces permeability in highly permeable subterranean zones using these methods.
Spaced sequential injection of polymer and cross-linking agent has also been shown to be ineffective because the gel components are displaced radially away from the well bore during injection by the spacer fluids. Gelation occurs too remotely from the well bore to effectively reduce permeability and control conformance in the formation.
There are other problems associated with using gels as permeability reducing compositions. It is difficult to create a partial plug with a gel because the gel is a continuous phase of uniform viscosity. A gel tends to either produce total shut-off of the formation or have no effect on permeability at all. Furthermore gels are often unstable at formation conditions of high temperature and varying pH.
The prior art also teaches conformance control in subterranean hydrocarbon-bearing formations using solid precipitates produced in situ. Metal hydroxide precipitates are created by contacting a polyvalent cation with a caustic solution in situ. The resulting precipitate reduces permeability in the formation as it settles out of solution in the porous zones to partially or completely plug them.
A simplified process is needed for reducing permeability in highly permeable zones of the formation, which minimizes the number of components and reactions required to effectively reduce permeability. A process is needed which allows the practitioner to implement either complete or partial plugging of the zones by reacting injected fluids in situ. It is desired that the reaction be rapid and relatively insensitive to formation temperature and pH and produces a stable permeability, reducing composition, which is not readily displaced during subsequent water flooding.